In order to increase RPM, the propeller needs rotational force applied from some source. With the plane at rest, the only source is provided by the motor. As it gains flight speed, airflow induced by forward flight can also induce prop rotation if the motor is stopped. The energy required to spin the propeller up with the motor off is going to be subtracted from the forward flight speed.

The same process is at work in helicopter auto rotation which allows for a soft landing in the event of engine failure. Helicopters have a free-wheeling unit that effectively disconnects the blades from the motor if it should fail and allows the blades to spin up and create enough lift to allow for a controlled, soft landing. If the blades were inhibited by the engine compression and transmission, it would fall like a brick.